Mobility, bioaccessibility, pollution assessment and risk characterization of potentially toxic metals in the urban soil of Lahore, Pakistan

The present study is based on the measurement of potentially toxic metal contents employing various extraction methodologies aimed at the evaluation of their mobility, bioaccessibility and bioavailability in the urban soil (n = 56) of Lahore, Pakistan. Selected metal levels in the soil were quantified using flame atomic absorption spectrometry. On the average basis, aqua regia and glycine extracts revealed comparatively higher contents for most of the metals; average concentrations of Fe, Mn, Zn, Pb, Cu, Cr, Co and Cd were found at 1566, 451.1, 114.8, 52.84, 39.15, 24.82, 12.59 and 3.953 mg/kg in aqua regia extract, while in glycine extract the metal levels were found at 579.6, 174.2, 74.72, 49.74, 19.28, 7.103, 4.692 and 3.357 mg/kg, respectively. However, Cd, Pb, Cu and Zn showed significantly higher mobility and bioavailability in the soil, while Co, Fe and Mn were least mobile/bioavailable. The pollution index was assessed in terms of enrichment factor and modified degree of contamination which revealed severe to significant contamination and anthropogenic enrichment of Cd, Pb, Cu and Zn. Multivariate analysis showed mostly anthropogenic contributions for Zn-Cu-Cr-Pb-Cd. Health risk assessment revealed relatively higher exposure of the metals through ingestion, while only minor contributions were noted for inhalation and dermal contact. Hazard quotient index was within the safe limit (< 1.0) in all soil extractions, thereby indicating no significant non-carcinogenic health risks. The incremental lifetime cancer risk for Cr (4.1E-06) through ingestion was comparatively higher than the safe limit which showed significant lifetime cancer risk to the local population.